Note: Descriptions are shown in the official language in which they were submitted.
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FUEL CELL CURRENT COLLECTOR
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present invention relate generally to fuel cells which
utilize a
current collector and/or a separator for the purpose of providing an
electronic flow path for
current generated by the fuel cell, to support the electrodes and electrolyte
holding member,
and to form the flow field for gas access to electrodes.
2. Description of Related Art
Conventional planar fuel cell stacks typically are comprised of a plurality of
fuel cell
sub-assemblies arranged in an electrical series relationship. A fuel cell sub-
assembly may
be formed of an anode electrode, a separator plate, and a cathode electrode.
An electrolyte
holding member is located between adjacent fuel cell sub-assemblies so as to
be in contact
with the anode and the cathode of adjoining fuel cell sub-assemblies. Another
approach is
to provide a plurality of membrane-electrode assemblies, or MEA's, with the
separators
located between adjacent MEA's. At assembly, the fuel cell stack is compressed
axially to
afford good intimate contact at each interface of the fuel cell stack to
establish the electronic
flow path for the electrons liberated by the electrochemical fuel cell
reaction.
The separator plate, being disposed between adjacent anodes and cathodes, is
required to be constructed from a conductive material. Typically, the basis
for selection of
material to construct the separator is a function of the operating
characteristics of the fuel cell
type. Each of the various fuel cell types has its particular electrolyte and
operating
temperature and provides various degrees of operating efficiencies. Typically,
fuel cells
which operate at low temperatures (-<400 C) may utilize a polymer separated
carbon
graphite for the separator material. Fuel cells which operate at temperatures
greater than
-400 C utilize stainless steels and ceramics as the separator material.
The separator plate of a conventional high temperature fuel cell stack serves
multiple
purposes.
= The separator acts as a housing for the reactant gasses to avoid leakage to
atmosphere
and cross-contamination of the reactants.
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= The separator acts as a flow field for the reactant gasses to allow access
to the
reaction sites at the electrode/electrolyte interfaces.
= The separator further acts as a current collector for the electronic flow
path of the
series connected fuel cells.
In many cases the separator is comprised of multiple components to achieve
these
purposes. Typically, three to four separate components, or sheets of material,
are needed
depending upon the flow configuration of the fuel cell stack. It is frequently
seen that one
sheet of material is used to provide the separation of anode/cathode gasses
while two
additional sheets are used to provide the flow field and current collection
duties for the anode
and the cathode sides of the separator. Another example of prior art is to use
one sheet of
ribbed or dimpled material to create the anode/cathode separation as well as
the flow fields.
Additional sheets of perforated material are used for current collection and,
in some
instances, to enhance the flow field cross sectional area.
There are three fundamental flow patterns of the reactant gasses which may be
applied to the separator to achieve varying objectives. The three patterns
consist of co-flow,
counter-flow, and cross-flow. Each of the three flow patterns introduces
varying degrees of
complexity to the design and construction of the separator and current
collectors. Low
temperature fuel cells, employing carbon graphite as the separator material,
often will utilize
a combination of the three fundamental flow patterns resulting in sinusoidal
flow paths or
"Z" patterns.
U.S. Patent No. 4,548,876 teaches the application of a "corrugated metallic
electron
collector" which "includes a plurality of corrugations therein". A preferred
embodiment
described in this patent utilizes particles within the metallic electron
collector to " provide
support for a respective catalyst (i.e. electrode) immediately adjacent to and
in contact with
the metallic electron collector". These collectors are adjacent to flat
"separators".
This approach has been further advanced through the application of an
additional
sheet metal component comprised of a perforated sheet positioned between the
"corrugated
metallic electron collector" and the respective electrode. These approaches
have proved to
be technically feasible; however, the material content of such structures is
economically
prohibitive, consisting of three to five sheet metal components of rather
significant
complexity.
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U.S. Patent No. 4,654,195 and U.S. Patent No. 5,531,956 among others teach the
application of "ribs" to the anode electrode of the fuel cell. This approach
is intended to
apply the flow field directly to the electrode. While technically effective,
typically the
material cost of the anode is greater than that of sheet metal used to
otherwise form an anode
flow field with the current collector and separator. Additionally, depth of
the ribs formed on
the anodes is insufficient for large area fuel cells requiring large cross-
sectional flow area.
Furthermore, excessive mechanical creep of ribbed anodes can result in poor
performance
of the fuel cell.
U.S. Patent No. 4,983,472 teaches the application of a "plurality of arches"
to the
current collector in a somewhat similar fashion as the above mentioned U.S.
Patent No.
4,548,876. However, the plurality of arches are distributed much more densely
and create a
finer degree of support to the electrodes thus eliminating the requirement for
supporting
particles or an additional perforated sheet metal component. This approach has
proven to be
technically successful but yet again has not reduced the component count of
the separator
plate below three sheets of material, i.e., two current collectors and one
separator sheet.
U.S. Patent No. 5,503,945 teaches the application of corrugations to the "main
plate"
of the separator and the use of perforated current collector for both the
anode and cathode.
This patent further teaches the integration of the current collector with its
respective
electrode. Additionally, this patent teaches the integration of the current
collector of either
the anode or the cathode with the peripheral sealing structure of the
separator and claims a
two piece separator with reduced material content and component count.
However, the
requirement for a current collector for the anode and for the cathode have not
been
eliminated. The active central area of the fuel cell typically constitutes the
far greater portion
of the area of the fuel cell relative to the peripheral sealing area.
Therefore, while component
count of the separator assembly has been reduced through integration of one of
the current
collectors with either the anode or with the cathode, the material content and
component
count of the separator as a whole has not appreciably been altered when
viewing the total
assembly. Furthermore, current collector/separator designs which utilize a
ribbed separator
and a nominally flat perforated current collector suffer from diminished cross-
communication of the reactant gas from one rib to adjoining ribs.
U.S. Patent No. 5,795,665 teaches the application of "a plurality of rows of
dimples"
to the separator plate and to the "current collector/active component
subassembly". Though
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resulting in modest reduction of material content the separator/current
collector component
count remains at three. This invention provides for cross-flow, co-flow, or
counter-flow of
reactant gasses utilizing three sheets of material.
U.S. Patent No. 5,811,202 teaches the application of ribs to an "anode field
plate" and
to a "cathode field plate" separated by a "flat middle plate". A perforated
current collector
is disposed between the cathode field plate and the cathode electrode
throughout the central
active area of the fuel cell. Again, as with the above mentioned U.S. Patent
No. 5,503,945,
the active central area of the fuel cell typically constitutes the far greater
portion of the area
of the fuel cell relative to the peripheral sealing area. Therefore, while
component count of
the separator assembly has been stated as being three, the component count of
the separator
as a whole when viewing the total assembly is four.
Material content of the separator/current collector assembly tends to be a
function of
two factors: the size of the active area of the fuel cell and the efficient
use of structural
forming materials. Fuel cells with areas exceeding about one square foot and
operating at or
near atmospheric pressures require reactant flow fields (i.e. anode and
cathode) with
sufficient cross-sectional area to prevent excessive pressure build-up at the
inlets. Excessive
pressure at the inlets relative to the outlets can create undesirable pressure
differentials that
may contribute to leakage of the reactants. As such, as cell area, and
consequently flow field
length, is increased to provide ever greater quantities of power output the
cross-sectional area
of the reactant flow fields also must increase. As it is the material of
construction of the
separator which forms the flow fields, material content of the separator rises
with the
increase in cross-sectional flow area. Excessive material content can be
controlled with
efficient use of the structural forming materials and the limitation of flow
field length.
Material thickness of the various current collector designs is governed by
several
factors which include corrosion rates of the various fuel cell environments as
well as the
mechanical constraints induced by the axial compressive load applied to the
fuel cell stack
and the unsupported spans of the flow field induced by the desire to maximize
the cross-
sectional flow area. As mentioned above, as cell area increases the cross-
sectional flow area
also is increased to accommodate the added reactant gas flow rate and
associated back
pressure. As a result fuel cells which seek large areas and/or operate at
atmospheric pressures
must provide relatively large cross-sectional flow area which tends to
manifest in wide
unsupported spans requiring robust current collector design.
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In some instances it has been found necessary to electroplate a stainless
steel alloy
with nickel for corrosion protection in anode current collector applications.
Nickel is very
stable in the anode environment of the molten carbonate fuel cell but does not
retain the same
degree of strength at operating temperatures as certain stainless steel
alloys. When the
5 current collector is configured to provide high degrees of structure to the
flow field, as with
U.S. Patent No. 4,983,472, a high strength alloy may be utilized to produce
the anode current
collector and typically is nickel plated following the forming process. Nickel
plating adds
significant expense to the manufactured cost of the current collector.
All of the above described prior art utilizes perforated sheet metal in one
form or
another to create the current collector of the fuel cell. These perforations
are arranged in
highly repetitive patterns to simplify manufacture as well as to maximize the
access of the
reactant gasses to the electrodes. The degree of reactant access to the
electrode is often
referred to as "percent open area" of the current collector. A reasonably
large percent open
area is needed to avoid choking the fuel cell electrochemical reaction which
diminishes
performance. However, large percentage of open area alone neglects the
electrodes
requirement for physical support against the stack axial compressive load. A
compromise is
made to limit percentage of open area while maintaining appropriately
dispersed electrode
support.
For example, molten carbonate fuel cell electrodes can sustain only modest
widths
of open and unsupported area of approximately. .10 inches. Additionally, the
performance of
molten carbonate fuel cells is diminished if the width of the areas of the
current collector
supporting the electrode exceeds approximately .10 inches. As a result the
typical molten
carbonate fuel cell (MCFC) current collector pattern of perforated openings
has a pitch in at
least one axis of approximately .20 inches. These factors become more
restrictive as
electrode thickness is reduced.
However, it has been observed that the electrodes are able to sustain
considerable
lengths of narrow unsupported areas. For example, the pattern described in the
above
mentioned # 4,983,472 patent where, although the width of the typical opening
is held at
.06", the length of the opening is .190". Another example is the description
in the above
mentioned # 5,811,202 patent which implies that the anode is supported only by
a ribbed
flow field implying significant lengths of narrow unsupported area.
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In some instances, as with the current collector described in U.S. Patent No.
4,983,472, the perforations are partial and the otherwise scrap metal is
utilized to form
the structure of the cross-sectic)nal flow area. As such, the manufacture of
these various
current collector designs typically requires specialized tooling operated in
press
machines of varying degrees of complexity. ln current collector patterns where
the
perforation is total and a through hole is produced a large quantity of scrap
metal is
generated during the manufacturing process, up to 35-40%. The complexity of
the
tooling and the generation of large quantity of scrap conspire to render the
various
designs as uneconomical at high production volunie anticipated for commercial
fuel cell
applications.
As mentioned, the typical highly repetitive patterns are intended to maximize
gas
access to electrodes for optin-iun7 fuel cell performance. However, the
various flow
patterns of the separator and ttle vagaries of the electrochemical fuel cell
process often
result in undesirable concentrations of electrochemical reactions. This tends
to create hot
spots and cold spots, or thennal gradients, in the f'uel cell which can
contribute to
premature failure of the fuel cell. It is well established in the art that
counter-flow of
reactant gasses in general provides the optimum distribution of the
electrochemical fuel
cell process and results in good current density distribution and reduced
thermal
gradients. In such a counter-flow approach the fiuel stream in each cell is
initially
directed to a region of the anode layer which coincides with the region of the
adjoining
cathode layer in which the oxidant stream has the lawest concentration of
oxygen. As
the fuel stream progresses through the fuel cell and approaches the exit it
has been
depleted of fuel and now coincides with the region of'the cathode in which the
oxidant
stream has the highest concentration of oxygen. It is further well established
in the art
2.5 that co-flow and cross-flow of reactant gasses provide for a less optimum
current density
distribution and tliermal gradients. It is additionally well established in
the art that the
current collector of a fuel cell is capable of diminishing the performance of
the fuel cell
if the current collector shields excessive areas of the electrodes from the
reactant gases.
BRIEF SUMMARY OF THE INVENTION
It is an object of an aspect of this invention to provide a plurality of flat
wires to
act as the current collector for a fuel cell which utilizes a ribbed separator
to form the
flow fields.
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It is a further object of an aspect of the present invention to provide a
current
collector for a fuel cell with long lengths of areas of alternately narrowly
supported
and narrowly unsupported electrodes.
It is a further object of an aspect of the present invention to provide a
current
collector for a fuel cell with lateral conduits for fluid communication
between
adjoining flow ribs of the separator.
It is a further object o1'an aspect of this invention to provide a current
collector
for a fuel cell which is simple to manufacture with little or no scrap
material
generated during the manufacturing process.
It is a further object of aii aspect of the present invention to provide a
current
collector that does not require electroplating of corrosion resistant
coatings.
It is yet a further object of an aspect of the present invention to provide a
current collector for a fuel cell such that the manufacturing tools may be
easily
modified or adjusted to alter the current collector pattern to selectively
concentrate or
disperse the fuel cell electro-r..hemical reaction to (lesirable locations to
optimize fuel
cell performance.
The preferred embodiment of the invention utilizes a plurality of parallel
flat
wires slit continuously from sheet metal and bonded to the face of an
electrode on the
side facing the respective flow field of the separator. The separator is
formed with
ribs. The flat wires, or strips, are preferably nan=ow and are preferably
spaced at
sufficient frequency, or pitch, as to provide optimum access of the reactant
gasses to
the electrodes as well as to provide optimum mechanical. support to the
electrodes.
The flat wires are preferably thin as to minimize material content and ease
manufacturing constraints )Y,ei: retain sufficient strength to react against
the
compressive seating forces applied to the fuel cell stack at assembly.
The flat wire current collectors are preferably continuously and
simultaneously slit from sheet nietal using a powered rotary slitting device
and spread
apart to the desired spacing through a combing device prior to an adhesive
bonding to
a preferably continuously tape cast electrode. The current collector/electrode
assembly may then be cut to desired length for installation to the ribbed
separator.
The flat wires will spari the peaks of the i-ibs of the separator and may be
arranged at any significant angle, other than parallel, to the ribs of the
separator that
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effectively supports the flat wire. Consequently, a secondary lateral conduit
is created
in the spaces between the flat wires which provides lateral fluid
communication of the
primary flow field conduit of the flow ribs for improved balancing of
pressures and
compositions of the reactant gasses.
According to one aspect of the present invention, there is provided a current
collector, for location between an electrode and a ribbed separator of a fuel
cell
having a plurality of separators, a plurality of electrodes, and a means of
supplying
reactants between each electrode and separator comprising: a plurality of flat
wires
having a planar surface contactable with an electrode; said plurality of flat
wires
having an opposing planar surface contactable with a ribbed separator.
According to a further aspect of the present invention, there is provided a
subassecnbly for use with a fuel cell comprising: a cathode, a plurality of
cathode
ettrrent collectors bonded to the cathode with an adhesive, an anode, a
plurality of
an.ode current collectors bonded to the anode with an adlzesive, and a
separator
contacting the pha'ality of cathode current collectors and the plurality of
anode current
collectors; wherein at least one of the plurality of cathode current
collectors and tlze
plurality of anode current collectors comprises flat wires.
According to a further aspect of the present invention, there is provided a
subassembly for use with a fuel cell comprising: a cathode, a plurality of
cathode
current collectors contacting the cathode, an anode, a plurality of anode
current
collectors contacting the anode, and a separator including adjacent ribs
contacting the
plurality of cathode current collectors aiid providing for lateral fluid
communication
between the adjacent ribs contacting the plurality of cathode current
collectors and
including adjacent ribs contacting the plurality of anode current collectors
and
providing for lateral fluid conunttnica.tion between the adjacent ribs
contacting the
plurality of anode current collectors; wherein at least one of the piurality
of cathode
ctirrent collectors and the plurality of anode current collectors comprises
flat wires.
According to a further aspect of the present invention, there is provided a
fuel
cell stack comprising a plurality of subassemblies, eaclz subassembly
comprisiug: a
cathode, a plurality of cathode current collectors bonded to the cathode with
an
adhesive, an anode, a plurality of anode current collectors bonded to the
anode with
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an adhesive, a separator contacting the plurality of cathode current
collectors aiid the
plurality of anode current collectors, and a plurality of electrolyte holding
members
disposed betwoen the subassemblies, wherein at least one of the pltt rality of
cathode
current collectors and the pluia.lity of anode current collectors comprises
flat wires.
According to a farther aspect of the present invention, there is provided a
subassembly for use with a fuel cell comprising: a catliode, a plurality of
cathode
current collectors contacting the cathode, an anode, a piurality of anode
currei3t
collectors contacting the anode, and a separator contacting the plurality of
calliode
cturent collectors and the plurality of anode current collectors; wherein at
least one of
the plttrality o f cathode cturent collectors and the plurality of anode
current collectors
comprises flat wires.
According to a fitrther aspect of the present invention, there is provided a
fuel
cell stack comprising a plurality of subassemblies, each sttbassembly
comprising: a
cathode, a plttrality of cathode current collectors contacting the cathode, an
anode, a
plurality of anode current collectors contacting the anode, and a separator
contacting
the plurality of cathode current collectors and the plurality of anode current
collectors,
and a plurality of electrolyte holding members disposed between the
subassemblies;
wherein at least one of the plurality of cathode current collectors and the
plurality of
anode current collectors comprises flat wires.
Upon viewing the figures aud reading the detailed description it will be
apparent to those skilled in the art that the current collector herein
described will be of
low material content, produced with little or no scrap, provide ease
ofmanufacture,
will not require corrosion resistant coatings, and may be easily manipulated
to
selectively concentrate or disperse the fuel cell electrochemical reaction to
desirable
locations for optimized fuel cell performance.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects of the invention will become more apparent upon
reading the following detailed description in conjunction with the
accompanying
drawings, in which:
FIG. 1 illustrates a portion of the central active area of the ctirrent
collector/electrode sub-assembly.
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FIG. 2 illustrates a portion of the central active urea of the
separator/curtent
collector/electrode assembly.
FIG. 3 illustrates a portion of the central active area of a plurality of
assemblies to form a fuel cell stack.
FIG. 4 illustrates the preferred m.anufactnring method for the invention.
FIG. 5 illustrates the invention manipulated to selectively concentrate or
disperse the electro-chemical fuel cell reaction.
DETAILED DESCRIPTION OF THE INVENTION
In Figure 1, a current collector/electrode sub-assembly 1 is shown. As
illustrated, the sub-assembly includes an electrode 2 and a plurality of flat
wires 3A,
3B, 3C,...which extend across the surface of electrode 2. The flat wires 3A,
3B,
3C,... are set at a pitch 4, possess a width 5, and possess a thiclrness 6. An
adhesive 7
is applied to one surface of flat wires 3A, 3B, 3C,... for the purpose of
bonding to the
electrode 2, The adhesive 7 is comprised of any of the adliesives laZown to
those
skilled in the art which may be removed following stack assembly by means of
oxidation at elevated tempcrature. The composition of the flat wires 3A, 3B,
3C,
sllottld consist of clemeiYts, or alloys of elements, known to
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remain stable in the fuel cell type to which they are to be installed. For
example, a molten
carbonate fuel cell (MCFC) may utilize alloy 316 SS for the cathode flat wire
and Nickel
200/201 for the anode flat wire. Experimentation has shown that anode flat
wire produced
from nickel 200/201 with a thickness of .005-.010 inches has sufficient
strength as to be
useable without a reinforcing substrate when spanning flow ribs with a span of
.060-.090
inches in fuel cell stacks with axial compressive loads of 15-20 PSI. Such a
construction
eliminates an expensive and environmentally unsound practice of electroplating
stronger
stainless steel alloys with nickel for corrosion protection.
In Figure 2, a current collector/anode electrode sub-assembly 1A and a current
collector/cathode electrode sub-assembly 1B are shown with a ribbed separator
10 to form
an assembly 11. An adhesive 7 is applied to the surfaces of sub-assemblies 1A
and 1B to
facilitate the assembly to the ribbed separator 10. As can be seen, the flat
wires 3A, 3B,
3C,... of both the anode assembly 1A and the cathode assembly 1B are parallel
to one
another and are not parallel to the flow ribs of the separator 10. A conduit
14 is created in the
space between each flat wire 3A, 3B, 3C....which provides fluid communication
between
adjacent ribs of the ribbed separator 10. It will be apparent to those skilled
in the art that the
pitch 4 and the width 5 of the flat wires 3A, 3B, 3C... may be altered to
varying degrees to
adjust for optimum mechanical support for, and optimum access of reactant
gasses 12 and
13 to, the electrodes 2. Experimentation has shown that a pitch 4 of .025-.045
inches and a
width 5 of .025-.045 inches provides good support to the electrode 2 as well
as good access
of reactant gasses. This combination of dimensions results generally in an
open area of the
current collector of about 50% but may be varied to a wide range. For example
the anode flat
wire current collector pitch and width may be altered to provide an open area
of perhaps 35%
to provide greater support to the anode electrode while maintaining sufficient
open area due
to the greater ease of the hydrogen fuel to access the catalytic reaction site
of the porous
anode electrode. Conversely, the well known molecular mass transport issues
associated with
the cathode and the oxidant gas may give cause to adjust pitch and width of
the cathode flat
wire current collector to that which would yield an increase in percentage of
open area.
Figure 3 shows a plurality of assemblies 11 disposed between a plurality of
electrolyte holding members 20 to form a portion of the central active area of
a fuel cell
stack. 15.
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Figure 4 illustrates a preferred method of manufacture for the flat wires 3A,
3B,
3C,... as well as a preferred method ofjoining them to the electrode 2. A
power rotary slitter
30 is provided with a plurality of discs 31A, 31B, 31C,...and 34A, 34B,
34C,... fitted to
opposing and counter rotating shafts 32, 33, respectively. The discs 31A, 31B,
31C,...are of
5 the appropriate diameter and thickness as to engage the discs 34A, 34B,
34C,...with
sufficient precision and strength as to slit the sheet meta135 dispensed from
a coil 36 to form
flat wires 3A, 3B, 3C,.... The thickness of the sheet metal 35 is equal to the
desired
thickness of the flat wire and, as has been stated, ranges from approximately
.005 inches to
approximately .010 inches. The flat wire is fed to a combing device 37 and
spread to the
10 desired pitch 4. A set of tensioning rolls 38 are positioned to prepare the
flat wire for
application of adhesive 7 and subsequent bonding to the continuously tape cast
electrode 2.
Those skilled in the art will be aware that high volume production of flat
wire by
conventional wire forming methods results in degrees of precision unnecessary
for the fuel
cell application. A tolerance of +/- .0002 inches is more than sufficient for
fuel cell
applications and is approximately the generally accepted tolerance for cold
rolled fine gauge
stainless steel sheet. A tolerance of +/- .00002 inches is the generally
accepted tolerance for
precision wire forming. So while it is technically feasible to produce the
flat wire current
collector with conventional wire forming technologies it is economically
prohibitive to use
any method other than slitting of sheet metal. Furthermore, while it is
feasible to gang spools
of pre-slit flat wire and dispense to the combing device for joining to the
electrode, it is
perhaps simpler to gang the slitters. An alternative to the application of the
adhesive as
shown may be the utilization of sheet metal with pre-applied thermal-sensitive
adhesive.
Those skilled in the art will see that the simplified method of manufacture of
the flat wire
current collector will result in little to no scrap relative to the various
perforated sheet metal
patterns from the prior art. Additionally, those skilled in the art will see
that the combination
of elimination of scrap and the spreading of the flat wire to the desired
pitch during
manufacture essentially results in 200% utilization of the material used to
produce the
current collector. In essence, a two piece separator is realized; one-half of
a piece for the
anode current collector, one piece for the ribbed separator, and one-half of a
piece for the
cathode current collector. Thus, reduction of material content is achieved.
Figure 5 illustrates how the invention may be manipulated to selectively
concentrate
or disperse the fuel cell electrochemical reaction. A fuel cell assembly 40 is
shown with a
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ribbed separator. The anode flow 41 and the cathode flow 42 are co-flow, or co-
current. The
pitch 4 and the width 5 of the initial flat wires 43A, 43B, 43C,... at the
entrance to the fuel
cell are varied in width and spacing relative to the flat wires 44A, 44B,
44C,... at the exit of
the fuel cell. The effect of the variation in pitch 4 and width 5 is to
selectively shield the
electrodes 2 from the reactants 41, 42 at a point when the concentrations of
fuel and oxidant
are at their highest in the fuel cell entrance. Similarly, at the exit the
pitch 4 and width 5
combine to open the electrodes 2 to the reactants 41, 42 at a point when the
concentrations
of fuel and oxidant are at their lowest in the fuel cell exit, the net effect
being to distribute
the electrochemical reaction for more evenly distributed power generation.
Similar
techniques are applicable for counter-flow separators as well as cross-flow
separators
utilizing two independently formed flow field plates as discussed in the prior
art.
In all cases it is understood that the above-described arrangements are merely
illustrative of the many possible specific embodiments which represent
applications of the
present invention. Numerous and varied other arrangements can be readily
devised in
accordance with the principles of the present invention without departing from
the spirit and
scope of the invention.
